From the Super Mario Wiki
Peach finding a Hidden Block
A Hidden Block is a block that contains coins, a star or random items in the Mario Party series. Its appearance changes with each game, sometimes resembling Brick Blocks, other times looking more like a small Treasure Chest. They only appear after landing on a space. It can give out anywhere from five to twenty coins or give out items such as Skeleton Keys, or even Stars.
Mario Party DS
In Mario Party DS, there are four kinds of Hidden Blocks.
- Brick Hidden Block: those blocks are the most common kind of blocks. They may contain coins, one Star or one Ztar.
- Coin Hidden Block: those blocks are unusual hidden blocks that contain coins. They look like the item Coin Block. Unlike regular Hidden Blocks, those blocks let the player get coins for 10 seconds, instead of 5.
- Star Hidden Block: those blocks are unusual hidden blocks that contain Stars. They look like the item Star Block. They contain 1-3 Stars.
- Item Hidden Block: very rare yellow blocks (with the Item Bag symbol on them) that contain 3 items.
There is also an one-time use item called a Block Sensor that makes a Hidden Block appear on the space the player lands on after using it.
- A Capsule similar to Hidden Blocks called the Coin Block Capsule appears in Mario Party 5. Like Hidden Blocks, they reward money, but unlike Hidden Blocks, their location is set on one space and visible to all.
- It is possible for the player to find out if the Hidden Block in Mario Party DS contains a Star (or Ztar) or coins. If the block appears right when the player lands on the space, the block holds coins. If the block appears later, it contains a Star or Ztar.